The present disclosure relates to systems for transporting and/or analyzing gas mixtures, and more particularly, to gas transport and analysis systems with photoionization detectors. Many industrial processes, such as semiconductor processing and pharmaceutical drying processes include the transport of chemicals as gas species in carrier gas streams. In such processes, it is generally desirable to know various characteristics of the gas, such as the partial pressure of the gas species in the carrier stream, in order to validate that the characteristics of the chemicals in the carrier stream are at the required levels for the particular process.
For instance, the semiconductor industry makes use of many deposition processes in which gas species are transported by a carrier gas. Semiconductor fabrication processes such as metal organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) each require the delivery of an organometallic precursor chemical at a particular partial pressure in a relatively inert carrier gas stream to a manufacturing process chamber. Control of the partial pressures of these precursors is often accomplished by relying on experimentally established recipes which are described, for example, in terms of pressures, temperatures, flows, durations, or other process characteristics. The instrumentation used to control such recipes can include temperature sensors, mass flowmeters, total pressure gauges, or other sensors. The actual amount of precursor materials being delivered to the process may be inferred from various measurements, and also controlled, for example, by adjustments to heater currents, flow control means, or other process parameters.
Some of these precursors are solids or liquids at room temperature, and are heated to establish the required partial pressures in the carrier gas as it passes through a precursor container. In these cases, it is also often necessary that the entire gas handling system be kept at elevated temperatures to prevent condensation of the precursor. All of the sensors used must tolerate the elevated temperatures required to prevent the precursor from condensing in the sensors themselves, as this would degrade the quality of the measurements being made and shorten the lifetimes of the sensors. Furthermore, as these sensors only measure total gas flow or total pressure of both species (precursor and carrier) in the stream, any change in the relative mixture amounts may go unnoticed. Changes that occur due to system faults such as failed heaters resulting in condensation of the precursor in the gas lines, or reduced evaporation of precursor due to decreasing surface area or channeling in the case of a solid precursor, or falling depth in the case of a liquid precursor, could be misinterpreted as changes in carrier flow, or overlooked altogether in the case of mixtures with low percentages of precursor.
Wajid et al. (U.S. Pat. No. 5,768,937) describes a system for measuring the makeup of a binary gas mixture of an MOCVD precursor in a carrier wherein the resonance frequency of an acoustic cell through which the binary mixture flows is measured. Since this frequency depends on the speed of sound in the gas that fills the cell, and as the speed of sound is directly related to the average molecular weight of the gas (as well as temperature and pressure), the composition of a mixture made up of two known components can be calculated based on knowledge of this resonance frequency. Such a system is somewhat complex, as it uses precision membranes as microphones and speakers, and requires tuned acoustic cavities with tight tolerances. Such systems also use elastomer or fluoroelastomer o-rings, and are prone to permeation of environmental gases through the o-rings and the outgassing of contaminants from the o-ring materials themselves, into the gas in the cell.
DeSisto et al. (U.S. Pat. No. 5,652,431) describes a sensor that measures the amount of metalorganic precursor in a gas stream wherein the gas stream flows through a UV-visible light absorption cell. UV-visible radiation enters the cell from one end and passes through some predetermined length of gas to the other end of the cell where it is collected. Some features of the spectral makeup of the collected radiation are then compared to those same features of the injected radiation, and the differences between them are indicative of the UV-vis absorption that can be attributed to the gas along the optical path. A similar system is described by Arno (U.S. Pat. No. 7,373,257 B2), except that rather than using UV-visible radiation, Arno describes measuring the partial pressure of precursors using the absorption by the gas mixture of different infrared wavelengths. Absorption based sensors require a large sensor size because the radiation must pass through a considerable amount of gas for accurate measurements to be obtained.
Leveson et al. (U.S. Pat. No. 4,413,185) describes the use of a photoionization detector in a gas chromatograph. Dean et al. (U.S. Pat. No. 7,046,012 B2) describes a photoionization detector used in a handheld environmental monitor. In both cases, the photoionization detector described comprises an ionization volume enclosed by insulating ionization chamber, often made of a dielectric such as a fluorocarbon-based plastic, with a plurality of electrodes positioned in the gas stream and in close proximity to the ionization process to enable collection of the ions that are made. The gas seals between the UV radiation sources in the photoionization detectors of the current art and their ionization chambers are made with either elastomer or fluoroelastomer o-rings or a tight fit through a hole in the ionization chamber. Such a photoionization detector is not suitable for use in high temperature environments.
Prior art sensors, such as those mentioned above, suffer from various limitations which make them inappropriate for use in certain applications, such as semiconductor processing or pharmaceutical drying. For instance, acoustic based sensors are incompatible with high temperature processing due the temperature sensitivity of their components. In addition, absorption based sensors typically require long lengths of transport gas to be analyzed, making them incompatible with the requirements of compact gas transport analyzers. Further, the sensors are also highly sensitive to the temperature of a process gas. As one example, the aforementioned sensors suffer from the limitation that the sensors themselves typically require materials of construction which make them incompatible with the high temperatures necessary for the transport of many modern organometallic precursors. As another example, speed of sound partial pressure sensors must account for the square root dependence of acoustic wave propagation on the temperature of the gas, thus mandating tight temperature control. Many acoustic receivers and transmitters also have strong and complex temperature dependencies. Infrared detectors must be kept thermally isolated from heated gas paths and excluded from heated zones to avoid unacceptably high thermal noise. Many UV sources such as UV-diodes have very low optical efficiency, and thus these devices must sink large amounts of waste heat to operate over acceptable lifetimes. Maintaining a cool emitter is made more difficult in proximity to high temperature gas paths and can be much more expensive in the case where the entire sensor is intended to be installed in a high temperature zone and thus requiring active cooling.
By way of further background, photoionization detectors may be used to measure the partial pressure of gas species having relatively low ionization energies in the presence of other gas species with higher ionization energies. For example, a sample of the gas mixture being analyzed may pass through a flow cell, and some sub-volume of the gas may be exposed to UV radiation of an energy high enough to cause ionization of the target gas species to be detected, but low enough not to cause ionization of the other species in the mixture. This ionization produces a population of positive ions the density of which is proportional to the number density of the target gas species. The proportionality is related to the ionization cross section of the species along with various geometrical factors describing the intersection of photon flux through the interrogated sample volume, and factors affecting the lifetime of the ions, such as collisions with walls. This number density can be related to the partial pressure of the target species by temperature with an equation of state such as the ideal gas law. Beyond the positive ions, the ionization process also results in the generation of free electrons that were removed from the target species gas molecules by the ionization process. An appropriately biased collector electrode positioned inside of the cell can be made to collect either the ions or, alternatively, the electrons by the choice of electrical biasing. Measurement of the current on this collector electrode is then, in principle, representative of the partial pressure of the ionized species.
In a photoionization detector, the collected current is dependent not only on the partial pressure of the ionized species, but also on the pressure of all species present. This is due to collisions between the ions and the gas in the chamber, as well as reduction of UV photon penetration length into the gas. Non-linearity in the collected ion current due to change in total pressure can be corrected for by incorporating knowledge of the total pressure in the system obtained from a suitable pressure gauge.
Photoionization detection technology may be used in gas chromatography, where the chemicals to be measured elute from a chromatographic column at times that are dependent on the gas species. The technology may also be used in hand-held sensors for detecting the presence and measuring the amount of various chemicals in the environment for the sake of environmental protection, health and safety, and tracking leaks, among others applications. In such a case, the gas mixture being analyzed may be near or above atmospheric pressure, and gas seals on the photoionization detectors may be made using various elastomers or fluoroelastomers. The gas mixture that has been analyzed by the photoionization detector may be subsequently exhausted as waste. While these types of materials and seals are generally acceptable for use in the typical photoionization detector applications, they are not tolerable in, e.g., semiconductor processing and some pharmaceutical drying applications that are far less tolerant of leakage either into or out of the sensor.